Sensitizable coated paper sheet adapted for photoelectrostatic reproduction

ABSTRACT

A PHOTOSENSITIVE PAPER SHEET WHICH IS USEFUL IN AN ELECTROPHOTOGRAPHIC PROCESS WHEREIN A LIQUID TONER IS EMPLOYED, SAID SHEET HAVING A COATED ON ONE SIDE THEREOF WHICH COMPRISES A FINELY DIVIDED, PHOTOCONDUCTIVE, INOGANIC PIGMENT DISPERSED IN A FILM-FORMING, ELECTRICALLY INSULATING BINDER MATERIAL, AND A COATING ON THE OTHER SIDE THEREOF WHICH ACTS AS A BARRIER COAT, TO PREVENT RETENTION OF THE LIQUID TONE ON THE BACK OF THE SHEET. A PHOTOSENSITIVE PAPER SHEET HAVING A BARRIER COATING ON BOTH SIDES THEREOF IS ALSO DISCLOSED.

Oct. 10, 1972 J J UBER ET AL 3,697,267

SENSITIZABLE COATED PAPER SHEET ADAPTED FOR PHOTOELECTROSTATIC REPRODUCTION Original Filed Jan. 27, 1966 x xx United States Patent O SENSITIZABLE COATED PAPER SHEET ADAPTED FOR PHOTOELECTROSTATIC REPRODUCTION Jay J. Uber, 67 Hitchcock St., Holyoke, Mass. 01040, and Johan F. Dirks, 12 Grandview St., South Hadley Falls, Mass. 01075 Continuation of application Ser. No. 541,866, Jan. 27 1966, which is a continuation-in-part of application Ser. No. 162,657, Dec. 28, 1961. This application Mar. 7, 1967, Ser. No. 621,365

Int. Cl. G03g 5 00,' C03c 5 D21h /00 U.S. Cl. 96-1.8 8 Claims ABSTRACT OF THE DISCLOSURE A photosensitive paper sheet which is useful in an electrophotographic process Wherein a liquid toner is employed, said sheet having a coating on one side thereof which comprises a finely divided, photoconductive, inorganic pigment dispersed in a film-forming, electrically insulating binder material, and a coating on the other side thereof which acts as a barrier coat, to prevent retention of the liquid toner on the back of the sheet. A photosensitive paper sheet having a barrier coating on both sides thereof is also disclosed.

This invention relates to a photosensitive paper sheet adapted for electrophotographic reproduction of images and more particularly, to a paper sheet carrying a photosensitive coating which is particularly well adapted for use in the production of a copy by an electrophotographic process which utilizes a. liquid toner. This application is a continuation-in-part of Ser. No. 162,657, filed Dec. 28, 1961 and now abandoned, and is a continuation of Ser. No. 541,866, filed Jan. 27, 1966, and also now abandoned.

In electrophotographic reproduction, an image is re, produced directly on a film or coating of a photoconductive pigment, such as zinc oxide, dispersed in an insulating matrix, such as a Synthetic resin, carried by a sheet of paper or other electn'cally conductive backing. In this process, the electrophotographic coating is given a negative electrostatic charge while protected from light, and then exposed to an optical light image of the subject matter to be reproduced. The electrostatic charge on the coating is discharged in the areas struck by light, while being retained in the areas protected from light, thereby producing an electrophotographic reproduction of the optical image.

Although specific reference is made to electrophotographic coatings containing zinc oxide as the photoconductive pigment, it will be understood that not only can other photoconductive pigments be substituted for the zinc oxide, but also other types of photoconductive coatings can be employed. Such other coatings may consist of organic photoeonductors dispersed or dissolved in a resinous binder, or they may consist either partially or entirely of photoconductive polymers.

In order to sensitiize efectively the electrophotograp'hic coating on the copy paper b'y the application of a blanket, negative electrostatic charge, it is necessary for the base sheet to have a minimal electrical conductivity.

Thus the photoconductive pigmented sheet in many respects performs similar functions to the Selenium drum of the well-tknown xerographic process.

Electrophotographic images are converted into visible images by either of two toning methods. In one toning method, the electrophotographic image is treated with a powdered, solid resinous toner carrying a positive electrostatic charge. The toner adheres to the areas of the coating carying a negative electrostatic charge to giwe a ice reproduction of the original optical image. This visible toner image is made permanent by sub'jectng it to heat to fuse the resinous powder.

In an alternate toning method, the electrophotographic image s treated with a colored, resinous liquid toner carried by 'volatile organic solvents. The excess toner is removed from the sheet by squeeze rolls and the volatile organic solvent permitted to evaporate to matke the image permanent.

A cursorily inspected zinc oxide coated electrophotographic sheet appears to be very simple, because a sheet of acceptable quality may seen to be easily obtained in view of many known coatings and coating processes. However, this impression is far removed from the facts. The performance requirements in general are as high, in their own way, as those in the selenium branch of xerography. At the same time, this high performance item must be mass produce'd in very large quantities at a very nominal cost.

The paper-base electrophotographic sheets which have heretofore been prod-uced have not been fully satisfactory in the production of copies by processes which utilize a liquid toner. Among other things, the back of the sheet carries unsightly and undesirable lines or streaks. These lines or strea'ks are caused by an uneven retention of the liquid toner by the back of the sheet. Further, the back of the sheet is darkened by the absorption of the toner, reducing the contrast in the image on the opposite side of the sheet.

Basically, a paper-base electrophotographie sheet should have a high degree of contrast between the print areas and the background or non-image areas. Obviously, any advance that Would allow the optimization of icontrast between the image and background would be an improvement, as the total appearanoe of the end product ideally should resemble the highest quality prints obtainable by any of the known printing methods.

'Prior art attempts to produce a high-quality sheet by optimizing this contrast have been exemplified by different approaches. British Pat. 873,080 shows an attempt to obtain contrast between image and background by using a bias voltage during the toning process. This patent focuses only on the surface toning problem because, as stated in the patent, the problem was believed to 'be associated with the surface electrophotographic layer of the base sheet.

Another approach in improving images in the liquid toning process of latent electrophotographic images is 'illustrated by 'Dirks U.S. Pat. 3,155,546, Nov. 3, 1964. In this patent the approach has been essentially through changes in apparatus and in the manner the basis voltage has been used in a modified apparatus.

Despite these meritorious modifications in an apparatus giving an improved image and background contrast, further improvements have been desired because certain problems have still persisted, such as streaking on the back of the sheet and other problems further discussed herein.

It is an object of this invention to provide a paperbase electrophotographic sheet which avoids the aforementioned undesirable problems such as streaking on the back or reverse surface when used to produce a copy by a process involving the use of the liquid toner, while being entirely satisfactory for use in the production of a copy by a process which utilizes a dry, 'powdered toner.

It is another object of this invention to provide a high contrast producing paper-base electrophotographic sheet which both avoids the undesirable streaking and darkening of the back or reverse surface when used to produce a copy by a process involving the use of a liquid toner and produces satisfactory copies under a wide range of atmospheric humidity conditions in processes using either a liquid or a dry toner.

Other objects of this invention and its various advantageous features will become apparent from the detailed description of this invention which follows.

The present invention is achieved by recognizing that instead of modifying the apparatus and/or using bias voltage, specific attention must be given to the construction of the electrophotographic paper sheet (copy paper).

In particular,` inv arriving at an acceptable copy paper ways must be found to prevent objectionable absorption of the binder for the zinc oxide into it. Moreover, the absorption of liquid toner in the back side of the copy paper during use `in the electrostatic copier must be pervented also.

This reduction in absorption must be accomplished without interfering with the normal handling of the sheet either during.` its manufacture or its use. For example,.thetreduction vin absorption on the face side of the sheet must notinterfere withthe adhesion of the zinc oxide coating over a wide range of temperature and humidity conditions.

The means used to reduce absorption of liquid toner on the back side of the sheet should neither interfere with'proper electrical grounding in the copier nor result in abnormal tackiness. Tackiness not only can result in multiple feeding in the electrostatic copier, but also it can cause the face Hside of one sheet to stick to the back side of another when in juxtaposition. Such -sticking can result in coating pick-off when the sheets are separated or when a roll of coated material is unwound. Furthermore, thematerials used to impart solvent resistance as well as those used to provide electrical conductivity must not transfer to. the zinc oxide coating on the face side when in face-to-back contact. To do so would destroy the photoconductivity of the zinc oxide coating and thus render the product useless.

It has now been found that in the design of this electrophotographic sheet, consideration must be given to the following:

(a).` Toner streaks on the back of the imaged sheet should be minimized, as determined by visual inspection.

(b) A mottled image effect should be minimized. Mottle is defined as non-uniformity in image density, and it is determined by visual observation of an electrostatic copy of a half-tone wedge having steps of .10 increment per,` step. The half-tone steps should be reproduced withv uniform density through out the entire wedge area.

(c) Unsightly `toner deposits should not be present on the back of the sheet.

(d) Absorption 'of the toner liquid 'in the copy paper should be minimal. The hold-out of such liquids can be determined by weighing an 8%" x 11" sheet of the copy paper before imaging and after imaging. A sheet of the present invention will absorb less than 0.3 gram per .sheet. A sheet having a hold-out of less than 0.2 gram is preferred.

(e)` -Background (non-image area) density of the imaged sheet should not exceed a value of 0.10 on the Kodak Gray Scale (Series V:7-61-CH).

(f) |Image density should be above a value of 0.90 on the Kodak GrayScale, and a density value of more than 1.00 is preferred.

(g) The Gurley density of the` sheet to which the zinc oxde coatingis applied should be in the range of 1000 to 5000 secs./ 10 ccs. (high-density method) for proper toner liquid and resin solvent hold-out. A Gurley density of from about 1500 to 4500 secs./ 10 ccs. is preferred (h) The thickness uniformity of the zinc oxide coating will be` enhanced if the sheet to which it is applied has a Sheflield Smoothness of from about 50 cc./sec. to about 130 cc./sec.

The Gurley density and smoothness levels indicated can be attained by applying barrier coatings of the types described |later in this patent, followed by proper calendermg.

(i) The adhesion of the zinc oxide coating to the support sheet should be good. O'therwise this coating Will flake `offwor become dislodged readily. Consequently, it is important to select barrier coating materials that will permit satisfactory adhesion. The degree of the zinc oxide coating adhesion to the support sheet can be determined readily by the well known Scotch Tape test. In making this test, the tape is applied to the coating with enough pressure to insure good uniform contact. The tape is then pulled away from the coating. The amount of coating sticking to the tape is a measure of its adhesion to the support sheet.

(j) The copy paper should have minimal curl both before and after imaging. Under TAPPI Standard Conditions, no edge of an 81/2 X 11" sheet should be more than 0.50 inch above the flat horizontal surface on which it rests.

(k) The copy paper should be made in such a manner that fast and uniform dissipation of the electrostatic charge is achieved in areas exposed to light.

(l) The specific electrical resistivity of the sheet to which the zinc oxide coating is applied should be no higher than 1013 ohm-cm. at 15% relative humidity.

(m) The barrier coatings that are applied to enhance solvent hold-out should neither dissolve nor soften unduly in aliphatic hydrocarbons, aromatic hydrocarbons, halogenated aliphatic hydrocarbons or in mixtures thereof.

(n) Tacky coatings as well as those having objectionable odors should be avoided. The sheets of copy paper should slide over one another readily. There should be no sticking nor dragging that could result in multiple sheet pick-up in the copy machine.

(o) The barrier coated sheet should retain unchanged the same sensitivity to moisture as uncoated cellulosic paper.

In making the above tests, a properly adjusted SCM Model 33 electrostatic copier is used wherever applicable.

Thus, the present invention is achieved by designing an appropriate barrier coating which is applied at least to the back of the base sheet and in the preferred design is apvplied to both sides of the sheet. This barrier coating, be-

cause of its physical and chemical nature, allows the deposition thereon of a well bonded photosensitive zinc oxide coating that is smooth and uniform in thickness. By practicing this invention a copy paper has been produced which is free of streaky and blotchy patterns on the back side when exposed to liquid toners during image development, gives copy of higher contrast, and minimizes toner liquid absorption.

All of the problems defined above have been eitherv trix of a film-forming electrically insulating materialv which bonds the inorganic pigment particles to each other as well as to the outer surface of the sheet. This copy paper carries at least on its'reverse side a barrier coating of a film-forrning material which is capable of deposition from an aqueous medium and which is insoluble in or relatively resistant to softening by volatile aliphatic hydrocarbons, volatile aromatic hydrocarbons, volatile halogenated aliphatic hydrocarbons, or mixtures thereof.

Additionally, the objectives of this invention are accomplished When coatings are applied to both sides of the sheet used as the base for the photosenstive copy paper that are insoluble in volatile hydrocarbon solvents and volatile halogenated hydrocarbon solvents. Suitable coating materials in this class are casein, soy protein, starch, oxidized starch, Dow Resin 2611.7 (described below), locust bean gum, polyacrylamide, or any other natural or Synthetic resins that are relatvely unafected by volatile hydrocarbon and halogenated hydrocarbon solvents or mixtures thereof either because of their inherent re-' sistance to these solvents or because of cross-linking that may take place after the coatings are applied. The coatings may be pigmented or unpigmented. Pigmented coatings are preferred because often they are less tacky than unpigmented coatings. Also the pigment helps to improve the smoothness of the sheet. Clay is the preferred pigment from the standpoints of performance as well as cost, although calcium carbonate, satn white, and 'barytes also are useful.

This copy paper, in its preferred embodiment, carries an intermediate coating between the paper surface and the photosenstive coating.

In the preferred embodiment of this invention, the barrier coated base sheet is impregnated throughout with an electrically conductive material. This material may be an inorganic salt, a conductive organic compound, conductive pigment, or a combination of two or more of these materials. Examples of suitable inorganic salts are lithum chloride, calcium chloride, trisodium phosphate, zinc nitrate, and potassium carbonate. Examples of suitable pigments are conductive carbon black, conductive zinc oxide, cuprous iodide, and copper oxide.

The barrier coated base paper may contain from about 0.1% to about of either the conductive inorganic salts or the conductive organic compounds. On the other hand, if conductive pigments are used, their content in the copy paper may range from about 1% to about 20% of the 'barrier coated base paper weight, depending upon individual conductivity and specific gravity characteristics.

Suitable organic compounds such as those mentioned in Gess U.S. Pat. 3,216,853, Nov. 9, 1965, also may be employed. A particularly effective ingredient for rendering the substrate (including the base paper plus any barrier coating on it) electrically conductive in a uniform manner is a resin produced by Dow Chemical Company, known as Dow Resin X-2611.7, which is sold as an aqueous solution carrying approximately 281/2% film-forming solids. Dow Resin X-26l1.7 is reputedly a polymer of the formula where R is lower alkyl such as -CH3 and R1 may be lower alkyl or and which can be used as a replacement of from about by weight, to about 95%, by weight, of the filmforming material of the barrier coating, since it is itself a film-forming material which seals the sheet against the penetration of the volatile components of liquid toner compositions.

Although no attempt is made to give an explanation as to why the barrier coating on the back side of the photosenstive copy paper minimizes unsightly toner deposits (streaks and the like), it is thought that both electrical conductivity and solvent 'barrier properties play significant roles. This apparent coaction of diverse properties emphasizes the co-operating effect of the members of this combination and the achievements which have been eifected without detrimentally aifecting the overall performance of the copy paper in either the wet or the dry toning process.

The products made in accordance with this invention are diagrammatically illustrated by the accompanying drawing, in which like reference characters are used to refer to like parts whereever they occur. In the drawing:

FIG. 1 is a magnified, broken cross-section of photoelectrostatic sheet in accordance with this invention; and

FIG. 2 s a magnified, broken, cross-section of a preferred photoelectrostatic sheet in accordance with this invention.

Referring specifically to FIGS. 1 and 2, the numeral 1 designates a copy paper carrying on its surface a photosensitive layer 2, and on its reverse surface a barrier coating 3 of a film-forming material which is capable of deposition from an aqueous medium and which is insoluble in volatile aliphatic hydrocarbons, volatile aromatic hydrocarbons, volatile halogenated aliphatic hydrocarbons, or in mixtures thereof. The barrier coated sheet 1 may have a basis weight within the range of 30 lbs. to 70 lbs. per ream of 3000 square feet and will preferably have a basis weight within the range of 'about 35 lbs. to 'about 60 lbs. per ream of 3000 square feet.

It will be noted that the embodiment of this invention illustrated by FIG. 2 shows an intermediate layer 4 beneath the photosenstive coating. This intermediate layer 4, like layer 3 on the reverse surface of the sheet, preferably is depositied from an aqueous medium, and it is insoluble in volatile aliphatic hydrocarbons, volatile aromatic hydrocarbons, volatile halogenated aliphatic hydrocarbons, or in mixtures thereof.

The barrier coatings 3 and 4 may be, `for example, casein, soy protein, oxidized starch, or a water-soluble gum such as locust gum. Furthermore, these coatings may include a latex, such as a butadiene-styrene latex, in a proportion below that which renders the coatings Sensitive to volatile aliphatic hydrocarbons, volatile aromatic hydrocarbons, or volatile halogenated aliphatic hydrocarbons. These coatings may carry no pigment, be lightly pigmented, or may carry a normal pigment content. The pigment carried by these coatings may be, for example, barium sulfate, calcium carbonate, calcium sulfate, clay, or satin white. When a pigment is included in the coatings 3 and 4, it will ordinarily be present within the range of about 0.1 part to about 10 parts by weight for each part of the film-forming material therein. The examples which follow specifically illustrate coating compositions which are suitable for the deposition of the barrier coatings 3 land 4 on a paper sheet in the production of the photo- Sensitive copy paper in accordance with this invention. It will be understood that the composition of the barrier coating which is deposited by these various compositions is identical with the composition of the non-volatile components therein, i.e., all the ingredients except the water of the composition.

EXAMPLE I Oxidized starch coating Oxidized starch dispersion (25% solids lbs. dry

7 EXAMPLE II Enzyme converted starch coating Starch lbs 600 Amylolytic enzyme grams 180 Clay lbs .v 1900 Titanium dioxide lbs..- 100 Sodium silicate .lbs 20 Tetrasodium pyrophosphate lbs-- 4 Water lbs 2000 Alum or soda ash to adjust pH to 7.2-7.7 before cooking.

EXAMPLE III Casein coating Casein solution 1 gals 120 Calcium carbonate lbs 200 Clay lbs 1400 Trisodium phosphate lbs-- 50 Water gals 1 lThe casein solution had the following composition:

Caseln lhe 200 Borax the 20 Ammonia (28%) ....qts..- 2 Water to make total volume 120 gals.

EXAMPLE IV Soy protein coating Isolated soy protein (low viscosity) lbs.. 80 Coating clay lbs 80 Ammonia (26 Be.) lbs-- 11.2 Formaldehyde donor (Hexamine) Lbs 3.2 Defoamer lbs-- .08 Lubricant (50% dispersion) lbS-- 2.4

Water to make total volume of 60 gals.

These coating compositions may be applied .to one side of a paper sheet by the usual paper-mill coating technique in the production of the photosensitive copy paper illustrated by FIG.` 1, or to both sides of the sheet in the production of the photosensitive copy paper illustrated by FIG. 2. Thus, they may be applied on the paper machine or in a separate coating operation. The weight of coating applied to form either the layer 3 of FIG. l, or the films 3 and 4 of FIG. 2, Will fall within the range of about 3 bs. to about 12 lbs. per ream of 3000 square feet. The coating is preferably calendered, or even supercalendered, to give it a smooth surface. Alternatively, it may be applied by one of the casting techniques which produces a very smooth surface.

The photosensitive coating 2 of both FIGS. 1 and 2 consists of a powdered photoconductor dispersed in an insulating matrix of a film-forming material such as, for example, silicone resin, poly-vinyl chloride resin,| polyethylene, phenolic resins, polyester resin, methacrylate resin, polystyrene resin, styrenated alkyd resin, and vinylacetate-vinylchloride copolymer resin.

The powdered inorganic photoconductor used in the photosensitive coating 2 may be almost any photoconductor having a sutficiently high value of photoconductivity. For example, the photoconductive oxides, sulphides, selenides, tellurides and. iodides of cadmium, mercury, antimouy, bismuth, thallium, molybdenum, aluminum, lead and zinc may be used. Also, arsenic trisulfide, cadmium arsenide, lead chromate and Selenium are useful for this purposefMxturesof two or more photoconductors may be used.

The powdered inorganic photoconductor whichis particularly suitable for use in this film is zinc oxide. A satisfactory zinc oxide f'or this purpose is that sold under the trade name of Florence Green Seal No. 8. This zinc oxide` meets both ASTM specification D79- 44 and Federal specification 'IT-Z301, and has the following analysis:

The amount of the photoconductor which is included in the film 2 will depend upon its specific grafvity and can be varied over a relatively wide range. In the case of zinc oxide, about 1 part, by weight, of the film-forming insulating matrix may be used With an amount of zinc oxide within the range of about 2.6 parts, by weight, to about 10.0 parts, by weight, and We prefer to use about 1 part, by weight, of the film-forming material with an amount of zinc oxide within the range of about 2.8 parts, by weight, to about 7.0 parts, by weight.

The examples which follow specifically illustrate coating compositions which are suitable for the deposition of the photosensitive coatings 2 of the embodiments of this invention illustrated by FIGS. 1 and 2. It will be understood that the compoistions of the photosensitive coatings deposited by these various compositions are identical with the composition of the non-volatile components therein, i.e., all of the ingnedients except the volatile solvent or solvents in the composition.

EXAMPLE V Parts by weight Silicone resin solution (G.E.-SR-82) 26 Zinc oxide (Florence Green Seal No. 8) 39 Toluene 35 Total EXAMPLE VI Parts by weight Silicone resin solution (G.E.-SR-82) 17 Zinc oxide (Florence Green Seal No. 8) 50 Toluene 33 Total 100 The silicone resin solution used in this composition contained 60% solids, vby weight, and 40%, by weight, of xylene. The solution had a vscosity within the range of about 5 to 30 centipoises at 25 C. and was straw in color.

EXAMPLE VII Parts by weight Copolymer of n-butyl methacrylate and so-butyl The copolymer of n-butyl methacrylate and iso-butyl methacrylate used in this composition was prepared by the use of equal parts, by weight, of the two monomers and has a density of 1.05 and a refractive index at 25 C. of 1.4778.

9 'EXAMIPL'E IX Parts by Weight Polymerized iso-butyl methacrylate (Lucite 45) 12.25 Zinc oxide (Florence Green Seal No. 8) 43. Toluene 56.25

Total 1 1 1.5

EXAMPL-E X Parts by weight Polymerized iso-butyl methacrylate (Lucite 45 12.25 Zinc oxide (Florence Green Seal No. 8) 73.50 Toluene 85.75

Total 17 1.50

The polymerized iso-buty] methacrylate used in this composition had a density of 1.05, a refractive index of 1.477 and a dielectric constant of 2.5.

The styrene-butadiene copolymer used in this composition Was high in styrene, had ya softening point of 50 C.3 C., a solution viscosity within the range of about 160 to about 195 seconds as measured by a No. 4 Ford Cup on an Xylene solution oon'taining 331/a%, by weight, of the resin, a specific gravity of 1.05, a refractive index of 1.585, a specific resistivity of approximately 1016 ohmcm., and a dielectric constant of 2.56 at 1000 cycles.

EXAMPL'E XIII Percent by wt. Styrenated alkyd copolymer solution (Cycopol S-101-1 24.0 Zinc oXide (Florence Green Seal No. 8) 42.5 Xylene 33.5

Total 100.0

The styrenated alkyd copolymer resin solution used in this composition contained 50% 1%, by weight, of solids dissol'ved in petroleum spirits. The solution had a color of 7.0 (Gardner 1933), a viscosity at 25 C. of ZZ and weighed 7.6 lbs. per gallon. The solid resin had an acid number of 5. w

The photosensitive coatings 2, illustrated by FIGS. 1 and 2, may be applied by the use of any of the various types of paper coating machines adapted to handle organic solvent coatings. The dry weight of the coating applied will ordinarily fall within the range of about 10 lbs. to about 45 lbs. per ream of 3000 square feet. It wil lordinarily have a thickness within the range of about 0.3 mil to about 1.0 mil after solvent removal.

Images may be reproduced on the photosensitive sheets in accordance with this invention in the same manner that images have been reproduced on the prior art paperbase photosensitive sheets, either by a process using dry toning or liquid toning. The sheets are entirely suitable for use in a process in which dry toning is used, and are particularly advantageous in a liquid toning process.

In the foregoing, details and specific illustrations of compositions suitable for use in the production of the photosensitive copy paper in accordance with this invention have been given for the purpose of fully explaining the invention. However, it will be understood that many variations can be made in the details which hafve been given, without departing from the spirit of the invention or the scope of the following claims.

What we claim is:

1. A photosensitive paper sheet adapted for use in an electrophotographic process which utilzes a liquid toner to produce a visible image, one surface of said sheet having a coating thereon of a photoconductive, finely-divided, inorganic pigment dispersed throughout a matrix of a film-forming, electricaly insulating material which bonds the photoconductive inorganic pigment to said surface, the other surface of said sheet having a barrier coating thereon of a film-forming material which is capable of deposition from an aqueous medium and which is relatively impervious to volatile organic solvents selected from the class consisting of aliphatic hydrocarbons, aromatic hydrocarbons and halogen-substituted aliphatic hydrocarbons.

2. A photosensitive paper sheet as claimed in claim 1 wherein the coating on one surface of said sheet has a dry weight of from about 10 pounds to about 45 pounds per ream of 3000 square feet of paper and comprises finelydivided photoconductive zinc oxide dispersed throughout the matrix of the film-forming, electrically insulating material which bonds said photoconductive, zinc oxide to the surface of said sheet, said film-forming, electrically insulating material being selected from the class consisting of silicone resins, polyvinyl chloride resins, polyethylene, phenolic resins, polyester resins, methacrylate resins, polystyrene resins, styrenated alkyd resins, vinylacetate-vinylchloride copolymer resins and styrene-butadiene copolymer resins, said photoconductive zinc oxide being present in said coating in an amount of from about 2.6 to a'bout 10 parts by weight per 1 part by weight of said film-forming, electrically insulating material; and wherein the barrier coating on the other surface of said sheet has a. weight of from about 3 pounds to about 12 pounds per ream of 3000 square feet of paper and comprises a filmforming material selected from the class consisting of casein, soy protein, starch, oxidized starch, locust beam gum, polyacrylamide, a butadiene-styrene latex, and a polymeric resin containing units of the formula:

wherein R is a lower alkyl group and `R' is a lower alkyl group or group having the formula 3. A photosensitive paper sheet as claimed in claim 2 wherein the barrier coating comprises as an additional component at least one pigment selected from the class consisting of barium sulfate, calcium carbonate, calcium sulfate, clay and satin white, said pigment being present in an amount of from about 0.1 part by weight per part of a film-forming material in said 'barrier coating to about 10 parts by weight per part of the film-forming material in said barrier coating.`

` 4. A photosensitive paper sheet as claimed in claim 3 wherein the paper sheet and the barrier coating thereon are impregnated `with an electrically conductive material selected from the class consisting of inorgauic salts, conductive organic compounds and conductive pigments.

5. A photosensitive paper sheet adapted for use in an electrophotographic process which utilizes a liquid toner to produce a visible image, each surface of said sheet having a barrier coating thereon of a filrn-forming material which is capable of deposition from an aqueous medium and which is relatively impervious to volatile organic solvents selected from the class consisting of aliphatc hydrocarbons, aromatic hydrocarbons and halogen-substituted aliphatic hydrocarbons, one of the surfaces of said sheet having an additional exterior coating thereon of a photoconductive, nfinely-divided, inorganic pigment dispersed throughout a matrix of a film-forming, electrically insulating material which bonds the photoconductive, inorganic pigment to the intermediate barrier coating.

6. A photosensitive paper sheet as claimed in claim 5 Wherein each of the barrier coatings has a weight of from aboutv 3 pounds to about 12 pounds per ream of 3000 square feet of paper and comprises a film-forming material selected from the class consisting of casein, soy protein, starch, oxidized starch, locust bean gum, polyacrylamide, a butadiene-styrene latex, and a polymeric resin containing units of the formula:

wherein 'R is a lower alkyl group and R' is a lower alkyl group or group having the formula and wherein the additional exterior coating has a dry weight of from about 10 pounds to about 45 pounds per ream of 3000 square feet of paper and comprises finelydivided photoconductive zinc oxide dispersed throughout the matrix of the film-forming, electrically insulating material which bonds said photoconductive zinc oxide to the intermediate barrier coating, said film-forming, electrically insulating material bengselected from the class consisting of silicone resins, polyvinyl Chloride resins, polyethylene, phenolic resins, polyester resins, methacrylate resins, polystyrene resins, styrenated alkyd resins, vinylacetate-vinylchloride copolymer resins and styrene-butadiene copolymer resins, said photoconductive zinc oxide being present in said coating in an amount of from about 2.6 to about 10 parts by weight per 1 part by weight of said film-forming, electrically insulating material.

7. A photosensitive sheet as claimed in claim 6 wherein at least one of the barrier coatings comprises asan additional component at least one pigment selected from the class consisting of barium sulfate calcium carbonate, calcium sulfate, clay and satin white, said pigment being present in an amount of from about 0.1 part by weight per part of a film-forming material in said barrier coating to about 10 parts by weight per'part of the film-forming material in said barrier coating.

8. A photosensitive paper sheet as claimed in claim 7 wherein the paper sheet and the barrier coatings thereon are impregnated 'with an electrically conductive material selected from the class consisting of inorganic salts, conductive organic compounds and conductive pigments.

References Cited UNITED STATES PATENTS 2,233,142 2/1941 Musher 117-60 X 2,899,335 8/1959' Stranghan 117-37 2,997,387 8/1961 Tannenbaum 96-15 3,017,826 1/1962 Salzberg 101-149.2 3,1l6,147 12/1963 Uber et al 96--l.5 3,241,958 3/1966 Bornarth et al 96--1.8

FOREIGN PATENTS 873,080 7/1961 Great Britain 96 1 GEORGE F. LE'SMES, Primary Examiner M. B. WI'ITENBERG, Assistant Examiner U.S. Cl. X.R. 

